Back in 1997 when I was 15 years old I had a 'Saturday job' (except it was on Sundays) at the first gym/fitness centre in town. Then every business was closed on Sundays including the gym. My job was to clean the wet area consisting of a sauna, steam room, plunge pool and showers. I had a key to the place and worked alone.
It was normal to be nude in the wet area (always single sexed) and I worked nude as it would have soaked any clothing worn in the sauna and steam room. One Sunday I turned up for work to find the owner's wife, sister, au pair and two children waiting for me. The owner's wife explained that she tried to call me to come in late as they were wanting to have a family sauna. Ok, I said I would use the gym until they were finished. The owner's sister asked me to join the family sauna as a better way to pass my time. So that was that, my first mixed nude experience and I was hooked especially as it felt so natural and swept away the hypocrisy of why we have single sexed facilities.
Later, my nude freedom extended to playing squash at the gym but that is another story for later.